SSD or Hard Drive for Time Machine, and Which Drives a Mac Runs at Full Speed

What Time Machine does, and how often it writes

Time Machine is the backup tool built into macOS, and it runs itself: hourly backups for the past 24 hours, daily for the past month, weekly for every month before that, the oldest deleted when the disk is full. The first backup copies everything and, Apple says, might take longer than you expect; after that it copies only the files that have changed, so the hourly runs are small. It also keeps hourly local snapshots on the Mac itself for 24 hours, and the schedule resumes when the drive is plugged back in, so a MacBook that meets its drive once a day is still backed up.

Why a hard drive is fine for it

Nothing in that pattern needs speed: each run copies an hour of changes in the background. That is why the two deepest owner records on our portable storage list belong to mechanical drives, the WD Elements and the Seagate Portable. For a backup, capacity for the money matters more than speed, and a mechanical drive gives roughly twice as much of it as an SSD.

The habit to respect: a mechanical drive must sit still while it works, because knocking a spinning platter mid-write is the most common way to kill one, and a Time Machine drive is written to every hour it is attached. It belongs on the desk, not in the laptop bag.

Where an SSD helps

The first backup and a full restore. The two jobs that move everything at once, the only times you wait on the drive.
Being knocked. No moving parts, so it survives being carried while writing. If the drive travels with the MacBook, buy an SSD for that reason alone.
Silence and size. Nothing to hear when it wakes every hour, and the Samsung T7 is about the size of a credit card.

It does not improve the hourly backups; they copy only what changed, and a hard drive keeps up.

How big the drive should be

Apple's guidance is at least twice the storage capacity of the Mac: for a 1TB Mac, ideally 2TB. Time Machine keeps its hourly, daily and weekly versions while there is room and deletes the oldest when the disk fills, so a smaller disk simply keeps fewer months of history; the Mac tells you when it needs more space. On our list that favours the mechanical drives for a 1TB Mac: the WD Elements, WD My Passport and Seagate Portable are 2TB listings, where the three SSDs are 1TB, enough for a 512GB Mac and not beyond.

APFS versus exFAT, and why the drive ends up APFS

Apple's preferred format for a Time Machine disk is APFS or APFS Encrypted, which its Disk Utility guide describes as the file system used by macOS 10.13 or later. The same guide files exFAT, with MS-DOS (FAT), under formats compatible with Windows, and Time Machine cannot back up to a disk formatted for Windows. Many portable drives arrive that way, so when you pick one as the backup disk macOS offers to erase and reformat it. The erase is total: do it before there is anything on the drive to lose.

The port question: USB 3 at 10Gb/s versus Thunderbolt

The current MacBook Air has two Thunderbolt 4 (USB-C) ports, which Apple lists at up to 40Gb/s for both Thunderbolt 4 and USB 4. The SSDs on our list are USB 3 drives, not Thunderbolt or USB 4, so they get the port's USB 3 mode, which Apple's sheet for the M2 MacBook Air spells out as USB 3.1 Gen 2 at up to 10Gb/s. Samsung's T7 page calls that same 10Gb/s link USB 3.2 Gen 2, so the T7, and the SanDisk Extreme with its stated 1,050MB/s read, sit within what the port offers.

The Samsung T9 is rated at up to 2,000MB/s over USB 3.2 Gen 2x2, a 20Gb/s mode, and Samsung's footnote says the host and cable must support Gen 2x2 to get there. That mode is on neither Apple sheet: 40Gb/s for Thunderbolt 4 and USB 4, 10Gb/s for USB 3.1 Gen 2, nothing between. On a MacBook Air the T9 drops to the 10Gb/s link (backward compatible, per Samsung) and runs at much the same speed as the T7. Ten gigabits a second is 1,250 megabytes a second before overhead, so no drive labelled 2,000MB/s gets past that on a Mac.

One drive for backup and files, or two

Apple's answer is two: the Time Machine volume is reserved for backups and Apple says not to store other files on it. Disk Utility can add a second APFS volume to the same disk, but the two share the space and the twice-the-Mac guidance gets harder to meet. The plainer reason: one drive holding your only copy and your backup is one failure from losing both. The rule our storage page uses is three copies, two kinds of media, one kept elsewhere.

Encryption

A backup disk holds a copy of everything, so losing one matters. Time Machine handles this itself: turn on Encrypt Backup when you set up the disk and choose a password, which you will need to restore, so keep it in a password manager. That works on any drive, which is the note on the WD My Passport below: its hardware encryption does the same job, and its bundled backup software is the kind of extra our storage page notes is often Windows-only.

Which of our picks fits which Mac owner

A Mac that stays on a desk: the WD Elements 2TB. A plain mechanical drive with no software, and the deepest owner record on our storage page at an exceptional rating. It is USB 3.0 with a USB-A cable, so a MacBook Air, whose data ports are all USB-C, needs a USB-C cable or adapter. The Seagate Portable 2TB is the same formula with a USB-A cable, an owner record nearly as deep and a marginally lower rating; buy whichever costs less on the day.

The same desk, sensitive data: the WD My Passport 2TB, the Elements with 256-bit hardware encryption and backup software added; on a Mac, Time Machine's encryption covers the same ground and the Elements has the deeper owner record. Check the listing for the cable.

A MacBook whose drive travels with it: the Samsung T7 Portable SSD 1TB. Credit-card sized, an aluminium case, USB 3.2 Gen 2 (the 10Gb/s a MacBook Air delivers), nothing inside to knock, and the highest owner rating of any drive on the storage page. Its trade is capacity: 1TB meets Apple's guidance for a 512GB Mac only. Check the listing for the cable.

Outdoors, on site, in the rain: the SanDisk Extreme Portable SSD 1TB. A rubberised, IP-rated shell with a carabiner loop, a stated 1,050MB/s read the port can carry, and the deepest owner record of any portable SSD on Amazon UK. Bulkier than the T7, and its ruggedness is wasted on a desk. Amazon UK labels it the older model; the newer one needs the same Gen 2x2 port as the T9.

A household with a desktop PC that has a Gen 2x2 port: the Samsung T9 Portable SSD 1TB. Up to 2,000MB/s on that port, drop-resistant from three metres and a five-year warranty per the listing, and both USB-C to C and USB-C to A cables in the box. On a MacBook Air it runs at much the same speed as the T7 and costs more.

The seventh pick, the SanDisk Ultra Dual Drive, is a pocket flash drive with USB-C and USB-A ends for moving files between devices, not a backup drive.

Can I use an external SSD for Time Machine?

Yes. Apple asks for an external drive on a USB or Thunderbolt port, formatted APFS or APFS Encrypted, and does not distinguish between an SSD and a hard drive. An SSD makes the first backup and a full restore faster and survives being carried; it does not change the hourly backups, and it holds roughly half the capacity of a mechanical drive for the same money.

Does a Time Machine drive need to be twice the size of the Mac?

That is Apple's guidance, ideally: at least twice the Mac's storage capacity, so 2TB for a 1TB Mac. It governs how much history you keep, not whether backups work, because Time Machine deletes the oldest backups when the disk is full. The floor is a disk that holds everything on the Mac once.

Why is my portable SSD slower than advertised on a MacBook Air?

Because the advertised figure belongs to a port the Mac does not have. Drives labelled 2,000MB/s, like the Samsung T9, need USB 3.2 Gen 2x2, a 20Gb/s mode on no MacBook Air sheet: Apple lists 40Gb/s for Thunderbolt 4 and USB 4 and, on the M2 model, 10Gb/s for USB 3.1 Gen 2. A USB 3 drive gets that 10Gb/s link, 1,250MB/s before overhead, so the T9 settles at roughly the T7's speed, while the T7 and the SanDisk Extreme, Gen 2 drives, fit inside it. If one of those is slower still, suspect the cable, which Samsung's footnote names alongside the host.